The present invention relates to toroidal transformers and, in particular, to low leakage inductance transformers.
U.S. Pat. No. 7,009,486 discloses toroidal transformers constructed on printed circuit boards and is incorporated herein by reference. The transformers of the patent are all one-to-one transformers, suitable, for example, for isolation purposes.
Referring to FIG. 2, such a prior art transformer 10 is shown schematically with a center-tapped primary winding 12 formed from the turns 14, 16, 18, 20. A magnetic core 22 couples the winding 12 to the center-tapped secondary winding 24 formed from the turns 26, 28, 30, 32.
Referring to FIGS. 1 and 3, the transformer 10 may be advantageously implemented with an annular magnetic core 22; a printed circuit board 34 containing traces 14A, 16A, 18A, 20A forming first portions of the winding 12, and traces 26A, 28A, 30A, 32A forming first portions of the winding 24; and staple-like conductors staples 14A, 16A, 18A, 20A forming second portions of the winding 12 and staples 26A, 28A, 30A, 32A forming second portions of the winding 24.
The core 22 is enlaced by the staples 14A, 16B, 18B, 20B, 26B, 28B, 30B, 32B when they are electrically and mechanically connected to the board 34, for example, by soldering.
The board 34 may advantageously be of a multilayer type with for example, (see FIG. 3A) a conductor (e.g., trace 26A) shielded above and below by a wider conductor (e.g., traces 36). The traces may be, for example, twice as wide as the sandwiched trace.
Many power applications draw large current from only one polarity of a power supply at a time. As a result, the large current flow in the secondary of a transformer flows in the winding above the center tap for one half of the transformer's input cycle and flows in the winding below the center tap for the other half of the input cycle. Similarly, it is common to drive a transformer's primary using a push-pull circuit. This results in current flowing only in the winding above the primary's center tap for the first half of the push-pull cycle and then flowing in the winding below the center tap during the other half of the push-pull cycle.
The transformer 10 takes this into account to minimize leakage inductance. The staple 14B and the staple 16B; the staple 26B and the staple 28B; the staple 18B and the staple 20B; and the staple 30B and the staple 32B are located on opposite sides of the transformer 10. By using this symmetrical arrangement of the staples, the mutual inductances between turns that are carrying large currents at the same time are reduced.
Displacement current (for example, parasitic capacitive leakage) between the primary and secondary winding is another source of common mode current/noise.
By locating primary staples adjacent to corresponding secondary staples, adjacent staples are electrically moving in the same direction at the same time, thus minimizing displacement current. For example, staple 14B is adjacent staple 26B, staple 16B is adjacent staple 28B, staple 18B is adjacent staple 30B, and staple 20B is adjacent staple 32B.
Typically, the center taps of the transformer are static with respect to the transformer signals and therefore do not couple common mode current. This advantageously allows the wide traces 36 to be added to the board 34 above and below electrically moving traces. All of the traces 36 are connected to either the primary or the secondary center tap. The traces 36 can act as either an electrostatic shield or a ground return, further improving the performance of the transformer 10.